As a variable pressure control valve, a valve constructed as shown, for example, in FIG. 1 is known.
Stated briefly, a variable pressure control valve is known wherein a spool 2 is kept in pressure contact with a valve seat of a pressurized fluid inlet port 1 by the resilient force of a compression spring 3, and a pilot fluid pressure Pp is introduced into a pilot piston 4 which supports the spring 3. The arrangement is made such that the force exerted on the spool 2 to keep it in pressure contact with the valve seat of the pressurized fluid inlet port 1 is increased or decreased by varying the pilot pressure Pp so that pressure of hydraulic fluid flowing thereinto from the pressurized fluid inlet port 1 can be variably set.
In such a variable pressure control valve, since the compression spring 3 is directly compressed by the pilot piston 4, it is necessary for the pilot piston 4 to have a large pressure receiving area. This results in a larger overall size of the valve, the valve cannot be made compact.
Stated more specifically, since in such a variable pressure control valve, the compression spring 3 having a high compression spring load is compressed directly by the pilot piston 4 and the pilot pressure Pp exerted on the pilot piston 4 is generally 30 kg/cm.sup.2, the pressure receiving area of the pilot piston 4 needs to be increased in order to increase the urging force thereof. Thus, the diameter of the pilot piston 4 becomes larger, thus unavoidably making the overall size of the valve larger.
Stating concretely, if the diameter of the pressurized fluid inlet port 1 is designated by d, and the diameter of the pilot piston 4 by D.sub.1, then the spring load W.sub.1 set by the compression spring 3 is given by the following formula ##EQU1## wherein P denotes the pressure within the pressurized fluid inlet port.
To increase the pressure P set in this condition by P, it is required to set the spring load W.sub.1 at .pi./4d.sup.2 (P+.DELTA.P).
Further, the urging or pushing force F given by the pilot piston 4 will be .pi./4D.sub.1.sup.2 X Pp.
Therefore, the diameter D.sub.1 of the pilot piston 4 needs to be set at such a value which enables the valve to support the above-mentioned spring load W.sub.1.